Chromate Conversion Coatings

Abstract

CHROMATE CONVERSION COATINGS are formed on metal surfaces as a result of the chemical attack that occurs when a metal is immersed in or sprayed with an aqueous solution of chromic acid, chromium salts such as sodium or potassium chromate or dichromate, hydrofluoric acid or hydrofluoric acid salts, phosphoric acid, or other mineral acids. The chemical attack facilitates the dissolution of some surface metal and the formation of a protective film containing complex chromium compounds. A variety of metals and electrodeposited metal coatings, including zinc, cadmium, magnesium, and aluminum, can be chromate conversion coated. Several articles in this Volume contain details about the procedures used to apply chromate coatings to specific metals and metal coatings. The conversion coating of cadmium electrodeposits is discussed in the article "Cadmium Plating"; the articles "Surface Engineering of Aluminum and Aluminum Alloys" and "Surface Engineering of Magnesium Alloys" contain information relating to the application of chromate conversion coatings to these metals; and procedures for applying conversion coatings to electrodeposited zinc are described in the article "Zinc Plating." A comprehensive summary of literature sources related to chromate conversion coatings is available in a detailed review of surface treatments for aluminum alloys (Ref 1). In addition, processing and testing details for application of chromium conversion coatings to aluminum, magnesium, cadmium, copper, silver, Zmc X "X" "X "X" Source: Ref 2 Economical replacement for anodizing if abrasion resistance is not required Used to "touch-up" damaged areas on anodized surfaces Thin coatings prevent "spotting out" of brass and copper electrodeposits. No fumes generated during chemical polishing

Effect of Different Passivation Treatments on Alkali Zn-Ni Coatings: Corrosion Resistance and Adhesion Performance of Geomet 321 and ML Black Coatings

“…As a result of this reaction, zinc forms a barrier layer called zinc chromate, which is resistant to moisture and air [16]. Water is absorbed when the passivation layer is scratched or mechanically damaged, and the damaged area is repaired [17]. The zinc chromate layer prevents anionic oxidation of the substrate surface by preventing interaction with oxygen, thus supporting the durability and longevity of the zinc coating.…”

Erosion Wear Property of Surface Diffusion Alloyed Coatings on Magnesium Alloy

“…However, the used condition of magnesium alloys were always both corrosion and wear, and the interactions between them would accelerate the damage [2][3][4]. There are a number of possible coating technologies available for magnesium and its alloys such as electrochemical plating, conversion coatings, anodizing, gas-phase deposition processes, and laser surface alloying/cladding and organic coatings [5][6][7][8][9], each has its own advantages and disadvantages [10][11][12][13][14][15][16]. For example, laser surface alloying are expensive; chemical conversion, anodization and electrochemical plating involve a lot of environmental pressure; and an organic coating normally needs a chemical conversion or anodized coating to prime the substrate first.…”